"Aftermarket" is a term used to describe accessories that are not made by the factory. So for example, for your Honda Pilot, if you buy accessories for your car that are not actually made by Honda, then those are termed "aftermarket". Many people use the term "Pilot aftermarket accessories" specifically when referring to custom accessories for the Pilot, like performance accessories or custom appearance accessories. But even if you just replace your Pilot brake pads with OEM-grade factory-style brake pads that are not made by Honda, those too are aftermarket Pilot accessories. Over the course of a vehicle's life, it almost certainly will have aftermarket accessories installed in it at some point, even if the vehicle is never actually "modded out".